GPs are writing prescriptions for the latest, most expensive antibiotics - ignoring cheaper and better options, say researchers.
There are concerns that antibiotics will lose their effect
The British Pharmaceutical Conference heard one in eight prescriptions in one area were for more modern drugs, often breaching guidelines.
Experts say these drugs should be held back as a "last line of defence" - to cut the risk of bacterial resistance.
Doctors' leaders said there might be good reasons for GPs' choices.
Doctors in both hospitals and GP surgeries are under great pressure to curb their antibiotic use, as over-use can lead to the development of "superbugs" resistant to all but a few modern drugs.
In recent years, new classes of powerful antibiotics have been developed to counter this threat, and guidelines say these should be used sparingly and only on certain conditions and where earlier antibiotics have failed.
Guidelines not followed
Four out of five prescriptions for antibiotics are written in the GP surgery, and a team from Liverpool John Moores University wanted to check what they were using and how often.
They looked at computer records and found that 15% of all antibiotic prescriptions were for more modern, expensive drugs, and that guidelines designed to restrict their use were not being followed.
"The risks associated with this behaviour are that the drug may be unnecessary or inappropriate, increasing the potential of resistance and increasing costs to the primary care trust."
Dr Jim Kennedy, the Royal College of GPs Head of Prescribing, said that a more detailed picture of prescribing would be needed to identify a problem.
"We don't know if this is a couple of rogue prescribers, or whether there were good reasons why the guidelines weren't being followed.
"The problem with guidelines is that none of them perfectly fits every situation."
He said that it was possible that the overall downward trend in antibiotic prescribing in recent years might mean that the appropriate use of stronger, modern antibiotics appeared to increase, as GPs continued to use these appropriately in the treatment of more serious bacterial infections.
Another study presented to the conference in Manchester highlighted the difficulties faced by those trying to curb overuse of antibiotics - this time within hospitals.
Many patients undergoing surgery are given a handful of doses of antibiotics to cut their chances of developing an infection, but Sunderland University researchers found in 2005 that more than half were getting too many in nearby City Hospitals Sunderland.
One woman patient was reportedly given 81 doses of antibiotics rather than the three to five required.
The trust changed its electronic prescribing system so that doctors would have to write in a date past which no antibiotics would be given.
However, the conference was told that the result was that 74% of patients received too much antibiotic, rather than 55% before the change, as doctors allowed for too many days of treatment.
Lead researcher Dr Rachel Etherington: "We think that doctors are still being overcautious about the risk of hospital acquired infections - perhaps partly due to media coverage of 'superbugs'."